InternetNews talks to developers and vendors about the rise of Btrfs as a successor to Ext4. Though Ext4 adds extents, Chris Mason, Btrfs developer noted that BTRFS adds a number of other features beyond that. Among those features are items like snapshotting, online file consistency checks and the ability to perform fast incremental backups. BTRFS (pronounced better FS) is currently under development in an effort led by Oracle engineer Chris Mason. With the support of Intel, Red Hat, HP, IBM, BTRFS could become the engine that brings next generation filesystem capabilities to Linux.

And other features (like I/O priority) don't even need to be implemented by btrfs because they are already implemented in the generic block layer.

IO priority would still be better implemented in the filesystem itself if you value performance. That way, the scheduler can do its work better since it's aware about the state of the filesystem and its structures, can reorder things better than a generic layer unaware of anything and just getting a list of requests.